US20050161672A1 - Electro-optical device and electronic device - Google Patents

Electro-optical device and electronic device Download PDF

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Publication number
US20050161672A1
US20050161672A1 US11/074,687 US7468705A US2005161672A1 US 20050161672 A1 US20050161672 A1 US 20050161672A1 US 7468705 A US7468705 A US 7468705A US 2005161672 A1 US2005161672 A1 US 2005161672A1
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display device
formed
substrate
film
electrode
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US7701134B2 (en
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Shunpei Yamazaki
Jun Koyama
Kunitaka Yamamoto
Toshimitsu Konuma
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Semiconductor Energy Laboratory Co Ltd
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Semiconductor Energy Laboratory Co Ltd
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Priority to JP15878799 priority
Priority to US09/578,895 priority patent/US8853696B1/en
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Priority to US11/074,687 priority patent/US7701134B2/en
Publication of US20050161672A1 publication Critical patent/US20050161672A1/en
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    • HELECTRICITY
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    • H01L33/08Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a plurality of light emitting regions, e.g. laterally discontinuous light emitting layer or photoluminescent region integrated within the semiconductor body
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Abstract

An object of the present invention is to provide an EL display device having a high operation performance and reliability.
The switching TFT 201 formed within a pixel has a multi-gate structure, which is a structure which imposes an importance on reduction of OFF current value. Further, the current control TFT 202 has a channel width wider than that of the switching TFT to make a structure appropriate for flowing electric current. Morever, the LDD region 33 of the current control TFT 202 is formed so as to overlap a portion of the gate electrode 35 to make a structure which imposes importance on prevention of hot carrier injection and reduction of OFF current value.

Description

    BACKGROUND OF THE INVENTION
  • 1. Field of the Invention
  • The present invention relates to an electro-optical device, typically an EL (electroluminescence) display device formed by a semiconductor element (an element using a semiconductor thin film) made on a substrate, and to electronic equipment (an electronic device) having the electro-optical device as a display (also referred to as a display portion).
  • 2. Description of the Related Art
  • Techniques of forming a TFT on a substrate have been widely progressing in recent years, and developments of applications to an active matrix type display device are advancing. In particular, a TFT using a polysilicon film has a higher electric field effect mobility (also referred to as mobility) than a TFT using a conventional amorous silicon film, and high speed operation is therefore possible. As a result, it becomes possible to perform pixel control, conventionally performed by a driver circuit external to the substrate, by a driver circuit formed on the same substrate as the pixel.
  • This type of active matrix display device has been in the spotlight because of the many advantage which can be obtained by incorporating various circuits and elements on the same substrate in this type of active matrix display device, such as reduced manufacturing cost, small size, increased yield, and higher throughput.
  • Switching elements are formed by a TFT for each of the pixels in the active matrix display device, current control is performed by driver elements using the switching elements, and an EL layer (electroluminescence layer) is made to emit light. A typical pixel structure at this time is disclosed in, for example, in FIG. 1 of U.S. Pat. No. 5,684,365 (Japanese Patent Application Laid-open No. Hei 8-234683).
  • As shown in FIG. 1 of the US Patent, a drain of a switching element (T1) is connected to a gate electrode of a current control element (T2), and is also connected in parallel to a capacitor (Cs). The gate voltage of the current control element (T2) is maintained by the electric charge stored in the capacitor (Cs).
  • Conversely, when the switching element (T1) is non-selected, the electric charge leaks through the switching element (T1) if the capacitor (Cs) is not connected (the flow of current at this point is referred to as off current), and the voltage applied to the gate electrode of the current control element (T2) becomes unable to be maintained. This is a problem which cannot be avoided because the switching element (T1) is formed by a transistor. However, the capacitor (Cs) is formed within the pixel, and therefore this becomes a factor in reducing the effective luminescence surface area (effective image display area) of the pixel.
  • Further, it is necessary for a large current to flow in the current control element (T2) in order to luminesce the EL layer. In other words, the performance required of the TFT becomes entirely different in the switching element and the current control element. In this type of case, it is difficult to ensure the performance required by all of the circuits and element with only one TFT structure.
  • SUMMARY OF THE INVENTION
  • In view of the above conventional technique, an object of the present invention is to provide an electro-optical device having good operation performance and high reliability, and in particular, to provide an EL display device. Another object of the present invention is to increase the quality of electronic equipment (an electronic device) having the electro-optical device as a display by increasing the image quality of the electro-optical device.
  • In order to achieve the above objects, the present invention assigns TFTs having an optimal structure in view of the performance required by elements contained in each pixel of the EL display device. In other words, TFTs having different structures exist within the same pixel.
  • Specifically, an element which places the most importance on sufficiently lowering the value of the off current (such as a switching element) is given a TFT structure in which the importance is more on reducing the off current value rather than on high speed operation. An element which places the greatest importance on current flow (such as a current control element) is given a TFT structure in which the importance is more on current flow, and on controlling deterioration due to hot carrier injection, which becomes a conspicuous problem at the same time, rather than on reducing the value of the off current.
  • It becomes possible to raise the operating performance of the EL display device, and to increase its reliability, with the present invention by performing proper use of TFTs on the same substrate, as above. Note that the concepts of the present invention are not limited to a pixel portion, and that the present invention is characterized by the point of being able to optimize the TFT structure contained in the pixel portion and in a driver circuit portion for driving the pixel portion.
  • BRIEF DESCRIPTION OF THE DRAWINGS
  • In the accompanying drawings:
  • FIG. 1 is a diagram showing the cross sectional structure of the pixel portion of an EL display device;
  • FIGS. 2A and 2B are diagrams showing the top view and the composition, respectively, of the pixel portion of an EL display device;
  • FIGS. 3A to 3E are diagrams showing manufacturing processes of an active matrix type EL display device;
  • FIGS. 4A to 4D are diagrams showing manufacturing processes of an active matrix type EL display device;
  • FIGS. 5A to 5C are diagrams showing manufacturing processes of an active matrix type EL display device;
  • FIG. 6 is a diagram showing an external view of an EL module;
  • FIG. 7 is a diagram showing the circuit block structure of an EL display device;
  • FIG. 8 is an enlarged diagram of the pixel portion of an EL display device;
  • FIG. 9 is a diagram showing the element structure of a sampling circuit of an EL display device;
  • FIG. 10 is a diagram showing the composition of the pixel portion of an EL display device;
  • FIG. 11 is a diagram showing the cross sectional structure of an EL display device;
  • FIGS. 12A and 12B are diagrams showing the top view and the composition, respectively, of the pixel portion of an EL display device;
  • FIG. 13 is a diagram showing the cross sectional structure of the pixel portion of an EL display device;
  • FIG. 14 is a diagram showing the cross sectional structure of the pixel portion of an EL display device;
  • FIGS. 15A and 15B are diagrams showing the top view and the composition, respectively, of the pixel portion of an EL display device;
  • FIGS. 16A to 16F are diagrams showing specific examples of electronic equipment;
  • FIGS. 17A and 17B are diagrams showing external views of an EL module;
  • FIGS. 18A to 18C are diagrams showing manufacturing processes of a contact structure;
  • FIG. 19 is a diagram showing the laminate structure of an EL layer;
  • FIGS. 20A and 20B are diagrams showing specific examples of electronic equipment;
  • FIGS. 21A and 21B are diagrams showing the circuit composition of the pixel portion of an EL display device;
  • FIGS. 22A and 22B are diagrams showing the circuit composition of the pixel portion of an EL display device; and
  • FIG. 23 is a diagram showing the cross sectional structure of the pixel portion of an EL display device.
  • DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
  • Embodiment Mode
  • FIGS. 1 to 2B are used in explaining the preferred embodiments of the present invention. Shown in FIG. 1 is a cross sectional diagram of a pixel of an EL display device of the present invention, in FIG. 2A is its top view, and in FIG. 2B is a circuit composition. In practice, a pixel portion (image display portion) is formed with a multiple number of this type of pixel arranged in a matrix state.
  • Note that the cross sectional diagram of FIG. 1 shows a cross section cut along the line A-A′ in the top view shown in FIG. 2A. Common symbols are used in FIG. 1 and in FIGS. 2A and 2B, and therefore the three figures may be referenced as appropriate. Furthermore, two pixels are shown in the top view of FIG. 2A, and both have the same structure.
  • Reference numeral 11 denotes a substrate, and reference numeral 12 denotes a base film in FIG. 1. A glass substrate, a glass ceramic substrate, a quartz substrate, a silicon substrate, a ceramic substrate, a metallic substrate, or a plastic substrate (including a plastic film) can be used as the substrate 11.
  • Further, the base film 12 is especially effective for cases in which a substrate containing mobile ions, or a substrate having conductivity, is used, but need not be formed for a quartz substrate. An insulating film containing silicon may be formed as the base film 12. Note that the term “insulating film containing silicon”indicates, specifically, an insulating film that contains silicon, oxygen, and nitrogen in predetermined ratios such as a silicon oxide film, a silicon nitride film, or a silicon oxynitride film (denoted by SiOxNy).
  • Two TFTs are formed within the pixel here. Reference numeral 201 denotes a TFT functioning as a switching element (hereafter referred to as a switching TFT), and reference numeral 202 denotes a TFT functioning as a current control element for controlling the amount of current flowing to an EL element (hereafter referred to as a current control TFT), and both are formed by an n-channel TFT.
  • The field effect mobility of the n-channel TFT is larger than the field effect mobility of a p-channel TFT, and therefore the operation speed is fast and electric current can flow easily. Further, even with the same amount of current flow, the n-channel TFT can be made smaller. The effective surface area of the display portion therefore becomes larger when using the n-channel TFT as a current control TFT, and this is preferable.
  • The p-channel TFT has the advantages that hot carrier injection essentially does not become a problem, and that the off current value is low, and there are already reports of examples of using the p-channel TFT as a switching TFT and as a current control TFT. However, by using a structure in which the position of an LDD region differs, the problems of hot carrier injection and the off current value in the n-channel TFT are solved by the present invention. The present invention is characterized by the use of n-channel TFTs for all of the TFTs within all of the pixels.
  • Note that it is not necessary to limit the switching TFT and the current control TFT to n-channel TFTs in the present invention, and that it is possible to use p-channel TFTs for either the switching TFT, the current control TFT, or both.
  • The switching TFT 201 is formed having: an active layer comprising a source region 13, a drain region 14, LDD regions 15 a to 15 d, a high concentration impurity region 16, and channel forming regions 17 a and 17 b; a gate insulating film 18; gate electrodes 19 a and 19 b, a first interlayer insulating film 20, a source wiring 21, and a drain wiring 22.
  • As shown in FIG. 2A, the present invention is characterized in that the gate electrodes 19 a and 19 b become a double gate structure electrically connected by a gate wiring 211 which is formed by a different material (a material having a lower resistance than the gate electrodes 19 a and 19 b). Of course, not only a double gate structure, but a so-called multi-gate structure (a structure containing an active layer having two or more channel forming regions connected in series), such as a triple gate structure, may also be used. The multi-gate structure is extremely effective in lowering the value of the off current, and by making the switching TFT 201 of the pixel into a multi-gate structure with the present invention, a low off current value can be realized for the switching TFT.
  • The active layer is formed by a semiconductor film containing a crystal structure. In other words, a single crystal semiconductor film may be used, and a polycrystalline semiconductor film or a microcrystalline semiconductor film may also be used. Further, the gate insulating film 18 may be formed by an insulating film containing silicon. Additionally, a conducting film can be used for all of the gate electrodes, the source wiring, and the drain wiring.
  • In addition, the LDD regions 15 a to 15 d in the switching TFT 201 are formed so as not to overlay with the gate electrodes 19 a and 19 b by interposing the gate insulating film 18. This structure is extremely effective in reducing the off current value.
  • Note that the formation of an offset region (a region that comprises a semiconductor layer having the same composition as the channel forming regions, and to which a gate voltage is not applied) between the channel forming regions and the LDD regions is more preferable for reducing the off current value. Further, when a multi-gate structure having two or more gate electrodes is used, the high concentration impurity region formed between the channel forming regions is effective in lowering the value of the off current.
  • By thus using the multi-gate structure TFT as the switching TFT 201, as above, a switching element having a sufficiently low off current value is realized by the present invention. The gate voltage of the current control element can therefore be maintained for a sufficient amount of time (for a period from one selection until the next selection) without forming a capacitor (Cs), such as the one stated in the conventional example.
  • Namely, it becomes possible to eliminate the capacitor which causes a reduction in the effective luminescence surface area, and it becomes possible to increase the effective luminescence surface area. This means that the image quality of the EL display device can be made brighter.
  • Next, the current control TFT 202 is formed having: an active layer comprising a source region 31, a drain region 32, an LDD region 33, and a channel forming region 34; a gate insulating film 18; a gate electrode 35; the first interlayer insulating film 20; a source wiring 36; and a drain wiring 37. Note that the gate electrode 35 has a single gate structure, but a multi-gate structure may also be used.
  • As shown in FIGS. 2A and 2B, the drain of the switching TFT 201 is electrically connected to the gate of the current control TFT 202. Specifically, the gate electrode 35 of the current control TFT 202 is electrically connected to the drain region 14 of the switching TFT 201 through the drain wiring (also referred to as a connection wiring) 22. Further, the source wiring 36 is connected to an electric current supply wiring 212.
  • A characteristic of the current control TFT 202 is that its channel width is larger than the channel width of the switching TFT 201. Namely, as shown in FIG. 8, when the channel length of the switching TFT is taken as L1 and its channel width as W1, and the channel length of the current control TFT is taken as L2 and its channel width as W2, a relational expression is reached in which W2/L2≧5×W1/L1 (preferably W2/L2≦10×W1/L1). Consequently, it is possible for more current to easily flow in the current control TFT than in the switching TFT.
  • Note that the channel length L1 of the multi-gate structure switching TFT is the sum of each of the channel lengths of the two or more channel forming regions formed. A double gate structure is formed in the case of FIG. 8, and therefore the sum of the channel lengths L1 a and L1 b, respectively, of the two channel-forming regions becomes the channel length L1 of the switching TFT.
  • The channel lengths l1 and L2, and the channel widths W1 and W2 are not specifically limited to a range of values with the present invention, but it is preferable that W1 be from 0.1 to 5 μm (typically between 1 and 3 μm), and that W2 be from 0.5 to 30 μm (typically between 2 and 10 μm). It is preferable that ;1 be from 0.2 to 18 μm (typically between 2 and 15 μm), and that L2 be from 0.1 to 50 μm (typically between 1 and 20 μm) at this time.
  • Note that it is preferable to set the channel length L in the current control TFT on the long side in order to prevent excessive current flow. Preferably, W2/L2≧3 (more preferably W2/L2≧5). It is also preferable that the current flow per pixel is from 0.5 to 2 μA (better between 1 and 1.5 μA).
  • By setting the numerical values within this range, all standards, from an EL display device having a VGA class number of pixels (640×480) to an EL display device having a high vision class number of pixels (1920×1080) can be included.
  • Furthermore, the length (width) of the LDD region formed in the switching TFT 201 is set from 0.5 to 3.5 μm, typically between 2.0 and 2.5 μm.
  • The EL display device shown in FIG. 1 is characterized in that the LDD region 33 is formed between the drain region 32 and the channel forming region 34 in the current control TFT 202. In addition, the LDD region 33 has both a region which overlaps, and a region which does not overlap the gate electrode 35 by interposing a gate insulating film 18.
  • The current control TFT 202 supplies a current for making the EL element 203 luminesce, and at the same time controls the amount supplied and makes gray scale display possible. It is therefore necessary that there is no deterioration when the current flows, and that steps are taken against deterioration due to hot carrier injection. Furthermore, when black is displayed, the current control TFT 202 is set in the off state, but if the off current value is high, then a clean black color display becomes impossible, and this invites problems such as a reduction in contrast. It is therefore necessary to suppress the value of the off current.
  • Regarding deterioration due to hot carrier injection, it is known that a structure in which the LDD region overlaps the gate electrode is extremely effective. However, if the entire LDD region is made to overlap the gate electrode, then the value of the off current rises, and therefore the applicant of the present invention resolves both the hot carrier and off current value countermeasures at the same time by a novel structure in which an LDD region which does not overlap the gate electrode is formed in series.
  • The length of the LDD region which overlaps the gate electrode may be made from 0.1 to 3 μm (preferable between 0.3 and 1.5 μm) at this point. If it is too long, then the parasitic capacitance will become larger, and if it is too short, then the effect of preventing hot carrier will become weakened. Further, the length of the LDD region not overlapping the gate electrode may be set from 1.0 to 3.5 μm (preferable between 1.5 and 2.0 μm). If it is too long, then a sufficient current becomes unable to flow, and if it is too short, then the effect of reducing off current value becomes weakened.
  • A parasitic capacitance is formed in the above structure in the region where the gate electrode and the LDD region overlap, and therefore it is preferable that this region not be formed between the source region 31 and the channel forming region 34. The carrier (electrons in this case) flow direction is always the same for the current control TFT, and therefore it is sufficient to form the LDD region on only the drain region side.
  • Further, looking from the viewpoint of increasing the amount of current that is able to flow, it is effective to make the film thickness of the active layer (especially the channel forming region) of the current control TFT 202 thick (preferably from 50 to 100 nm, more preferably between 60 and 80 nm). Conversely, looking from the point of view of making the off current value smaller for the switching TFT 201, it is effective to make the film thickness of the active layer (especially the channel forming region) thin (preferably from 20 to 50 nm, more preferably between 25 and 40 nm).
  • Next, reference numeral 41 denotes a first passivation film, and its film thickness may be set from 10 nm to 1 μm (preferably between 200 and 500 nm). An insulating film containing silicon (in particular, preferably a silicon oxynitride film or a silicon nitride film) can be used as the passivation film material. The passivation film 41 plays the role of protecting the manufactured TFT from contaminant matter and moisture. Alkaline metals such as sodium are contained in an EL layer formed on the final TFT. In other words, the first passivation film 41 works as a protecting layer so that these alkaline metals (mobile ions) do not penetrate into the TFT. Note that alkaline metals and alkaline-earth metals are contained in the term ‘alkaline metal’ throughout this specification.
  • Further, by making the passivation film 41 possess a heat radiation effect, it is also effective in preventing thermal degradation of the EL layer. Note that light is emitted from the base 11 side in the FIG. 1 structure of the EL display device, and therefore it is necessary for the passivation film 41 to have light transmitting characteristics.
  • A chemical compound containing at least one element selected from the group consisting of B (boron), C (carbon), and N (nitrogen), and at least one element selected from the group consisting of Al (aluminum), Si (silicon), and P (phosphorous) can be given as a light transparent material possessing heat radiation qualities. For example, it is possible to use: an aluminum nitride compound, typically aluminum nitride (AlxNy); a silicon carbide compound, typically silicon carbide (SixCy) ; a silicon nitride compound, typically silicon nitride (SixNy); a boron nitride compound, typically boron nitride (BxNy); or a boron phosphate compound, typically boron phosphate (BxPy). Further, an aluminum oxide compound, typically aluminum oxide (AlxOy), has superior light transparency characteristics, and has a thermal conductivity of 20 Wm−1K−1, and can be said to be a preferable material. These materials not only possess heat radiation qualities, but also are effective in preventing the penetration of substances such as moisture and alkaline metals. Note that x and y are arbitrary integers for the above transparent materials.
  • The above chemical compounds can also be combined with another element. For example, it is possible to use nitrated aluminum oxide, denoted by AlNxOy, in which nitrogen is added to aluminum oxide. This material also not only possesses heat radiation qualities, but also is effective in preventing the penetration of substances such as moisture and alkaline metals. Note that x and y are arbitrary integers for the above nitrated aluminum oxide.
  • Furthermore, the materials recorded in Japanese Patent Application Laid-open No. Sho 62-90260 can also be used. Namely, a chemical compound containing Si, Al, N, O, and M can also be used (note that M is a rare-earth element, preferably an element selected from the group consisting of Ce (cesium), Yb (ytterbium), Sm (samarium), Er (erbium), Y (yttrium), La (lanthanum), Gd (gadolinium), Dy (dysprosium), and Nd (neodymium)). These materials not only possess heat radiation qualities, but also are effective in preventing the penetration of substances such as moisture and alkaline metals.
  • Furthermore, carbon films such as a diamond thin film or amorphous carbons (especially those which have characteristics close to those of diamond; referred to as diamond-like carbon) can also be used. These have very high thermal conductivities, and are extremely effective as radiation layers. Note that if the film thickness becomes larger, there is brown banding and the transmissivity is reduced, and therefore it is preferable to use as thin a film thickness (preferably between 5 and 100 nm) as possible.
  • Note that the aim of the first passivation film 41 is in protecting the TFT from contaminating matter and from moisture, and therefore it must made so as to not lose this effect. A thin film made from a material possessing the above radiation effect can be used by itself, but it is effective to laminate this thin film and a thin film having shielding properties against alkaline metals and moisture (typically a silicon nitride film (SixNy) or a silicon oxynitride film (SiOxNy)). Note that x and y are arbitrary integers for the above silicon nitride films and silicon oxynitride films.
  • Reference numeral 42 denotes a color filter, and reference numeral 43 denotes a fluorescent substance (also referred to as a fluorescent pigment layer). Both are a combination of the same color, and contain red (R), green (G), or blue (B). The color filter 42 is formed in order to increase the color purity, and the fluorescent substance 43 is formed in order to perform color transformation.
  • Note that EL display devices are roughly divided into four types of color displays: a method of forming three types of EL elements corresponding to R, G, and B; a method of combining white color luminescing EL elements with color filters; a method of combining blue or blue-green luminescing EL elements and fluorescent matter (fluorescing color change layer, CCM); and a method of using a transparent electrode as a cathode (opposing electrode) and overlapping EL elements corresponding to R, G, and B.
  • The structure of FIG. 1 is an example of a case of using a combination of blue luminescing EL elements and a fluorescent substance. A blue color emitting luminescence layer is used as the EL element 203 here, light possessing blue color region wavelength, including ultraviolet light, is formed, and the fluorescent substance 43 is activated by the light, and made to emit red, green, or blue light. The color purity of the light is increased by the color filter 42, and this is outputted.
  • Note that it is possible to implement the present invention without being concerned with the method of luminescence, and that all four of the above methods can be used with the present invention.
  • Furthermore, after forming the color filter 42 and the fluorescent substance 43, leveling is performed by a second interlayer insulating film 44. A resin film is preferable as the second interlayer insulating film 44, and one such as polyimide, polyamide, acrylic, or BCB (benzocyclobutane) may be used. An inorganic film may, of course, also be used, provided that sufficient leveling is possible.
  • The leveling of steps due to the TFT by the second interlayer insulating film 44 is extremely important. The EL layer formed afterward is very thin and therefore there are cases in which poor luminescence is caused by the existence of a step. It is therefore preferable to perform leveling before forming a pixel electrode so as to be able to form the EL layer on as level a surface as possible.
  • Furthermore, it is effective to form an insulating film having a high thermal radiation effect (hereafter referred to as a thermal radiation layer) on the second interlayer insulating film 44. A film thickness of 5 nm to 1 μm (typically between 20 and 300 nm) is preferable. This type of thermal radiation layer functions so that the heat generated by the EL element is released, so that heat is not stored in the EL element. Further, when formed by a resin film, the second interlayer insulating film 44 is weak with respect to heat, and the thermal radiation layer works so as not to impart bad influence due to the heat generated by the EL element.
  • It is effective to perform leveling of the TFT by the resin film in manufacturing the EL display device, as stated above, but there has not been a conventional structure which considers the deterioration of the resin film due to heat generated by the EL element. It can therefore be said that the formation of the thermal radiation layer is extremely effective in resolving this point.
  • Furthermore, provided that a material which is not permeable to moisture, oxygen, or alkaline metals (a material similar to that of the first passivation film 41) is used as the thermal radiation layer, then it can also function as a protecting layer in order that alkaline metals within the EL layer do not diffuse toward the TFT, at the same time as preventing deterioration of the EL element and the resin film due to heat, as above. In addition, the thermal radiation layer also functions as a protecting layer so that moisture and oxygen do not penetrate into the EL layer from the TFT.
  • In particular, provided that the thermal radiation effect is desired, a carbon film such as a diamond film or a diamond-like carbon film is preferable, and in order to prevent penetration of substances such as moisture, it is more preferable to use a lamination structure of a carbon film and a silicon nitride film (or a silicon oxynitride film).
  • A structure in which TFT side and EL element side are segregated by an insulating film which has a high radiation effect and is capable of shielding moisture and alkaline metal, is thus effective.
  • Reference numeral 45 denotes a pixel electrode (EL element anode) made from a transparent conducting film. After opening a contact hole in the second interlayer insulating film 44 and in the first passivation film 41, the pixel electrode 45 is formed so as to be connected to the drain wiring 37 of the current control TFT 202.
  • An EL layer (an organic material is preferable) 46, a cathode 47, and a protecting electrode 48 are formed in order on the pixel electrode 45. A single layer structure or a lamination structure can be used as the EL layer 46, but there are many cases in which the lamination structure is used. Various lamination structures have been proposed, combinations of layers such as a luminescence layer, an electron transporting layer, an electron injecting layer, a hole injecting layer, and a hole transporting layer, but any structure may be used for the present invention. Doping of a fluorescent pigment into the EL layer may also be performed, of course. Note that a luminescing element formed by a pixel electrode (anode), an EL layer, and a cathode is referred to as an EL element throughout this specification.
  • All already known EL materials can be used by the present invention. Organic materials are widely known as such materials, and considering the driver voltage, it is preferable to use an organic material. For example, the materials disclosed in the following U.S. Patents and Japanese patent applications can be used as the organic EL material:
  • U.S. Pat. No. 4,356,429; U.S. Pat. No. 4,539,507; U.S. Pat. 4,720,432; U.S. Pat. No. 4,769,292; U.S. Pat. No. 4,885,211; U.S. Pat. No. 4,950,950; U.S. Pat. No. 5,059,861; U.S. Pat. No. 5,047,687; U.S. Pat. No. 5,073,446; U.S. Pat. No. 5,059,862; U.S. Pat. No. 5,061,617; U.S. Pat. No. 5,151,629; U.S. Pat. No. 5,294,869; U.S. Pat. No. 5,294,870; Japanese Patent Application Laid-open No. Hei 10-189525; Japanese Patent Application Laid-open No. Hei 8-241048; and Japanese Patent Application Laid-open No. Hei 8-78159.
  • Specifically, an organic material such as the one shown by the following general formula can be used as a hole injecting layer.
    Figure US20050161672A1-20050728-C00001
  • Here, Q is either N or a C—R (carbon chain); M is a metal, a metal oxide, or a metal halide; R is hydrogen, an alkyl, an aralkyl, an aryl, or an alkalyl; and T1 and T2 are unsaturated six member rings including substituent such as hydrogen, alkyl, or halogen.
  • Furthermore, an aromatic tertiary amine can be used as an organic material hole transporting layer, preferably including the tetraaryldiamine shown by the following general formula.
    Figure US20050161672A1-20050728-C00002
  • In formula 2 Are is an arylene group, n is an integer from 1 to 4, and Ar, R7, R8, and R9 are each various chosen aryl groups.
  • In addition, a metal oxynoid compound can be used as an organic material EL layer, electron transporting layer, or electron injecting layer. A material such as that shown by the general formula below may be used as the metal oxinoid compound.
    Figure US20050161672A1-20050728-C00003
  • It is possible to substitute R2 through R7, and a metal oxinoid such as the following can also be used.
    Figure US20050161672A1-20050728-C00004
  • In formula 4, R2 through R7 are defined as stated above; L1 through L5 are carbohydrate groups containing from 1 to 12 carbon elements; and both L1 and L2, or both L2 and L3 are formed by benzo-rings. Further, a metal oxinoid such as the following may also be used.
    Figure US20050161672A1-20050728-C00005
  • It is possible to substitute R2 through R6 here. Coordination compounds having organic ligands are thus included as organic EL materials. Note that the above examples are only some examples of organic EL materials which can be used as the EL material of the present invention, and that there is absolutely no need to limit the EL material to these.
  • Furthermore, when using an ink jet method for forming the EL layer, it is preferable to use a polymer material as the EL material. Polymer materials such as the following can be given as typical polymer materials: polyparaphenylene vinylenes (PPVs); and polyfluorenes. For colorization, it is preferable to use, for example, a cyano-polyphenylene vinylene in a red luminescing material; a polyphenylene vinylene in a green luminescing material; and a polyphenylene vinylene and a polyalkylphenylene in a blue luminescing material. Regarding organic EL materials which can be used in an ink-jet method, all of the materials recorded in Japanese Patent Application Laid-open No. Hei 10-012377 can be cited.
  • Furthermore, a material containing a low work coefficient material such as magnesium (Mg), lithium (Li), cesium (Cs), barium (Ba), potassium (K), beryllium (Be), or calcium (Ca) is used as the cathode 47. Preferably, an electrode made from MgAg (a material made from Mg and Ag at a mixture of Mg:Ag=10:1) may be used. In addition, a MgAgAl electrode, a LiAl electrode, and a LiFAl electrode can be given as other examples. Further, the protecting electrode 48 is an electrode formed in order to be a protecting film against moisture from external to the cathode 47, and a material containing aluminum (Al) or silver (Ag) is used. The protecting electrode 48 also has a heat radiation effect.
  • Note that it is desirable to form the EL layer 46 and the cathode 47 in succession, without exposure to the atmosphere. In other words, no matter what type of lamination structure the EL layer and the cathode contain, it is preferable to form everything in a multi-chamber (also referred to as a cluster tool) type deposition device in succession. This is in order to avoid the absorption of moisture when the EL layer is exposed to the atmosphere because if an organic material is used as the EL layer, then it is extremely weak with respect to moisture. In addition, not only the EL layer 46 and the cathode 47, it is even better to form all the way through the protecting electrode 48 in succession.
  • The EL layer is extremely weak with respect to heat, and therefore it is preferable to use vacuum evaporation (in particular, an organic molecular beam evaporation method is effective in that it forms a very thin film, on the molecular order level), sputtering, plasma CVD, spin coating, screen printing, or ion plating as the film deposition method. It is also possible to form the EL layer by an ink-jet method. For the ink jet method there is a bubble jet method using cavitation (refer to Japanese Patent Application Laid-open No. Hei 5-116297), and there is a piezo method using a piezo element (refer to Japanese Patent Application Laid-open No. Hei 8-290647), and in view of the fact that organic EL materials are weak with respect to heat, the piezo method is preferable.
  • Reference numeral 49 denotes a second passivation film, and its film thickness may be set from 10 nm to 1 μm (preferable between 200 and 500 nm). The object of forming the second passivation film 49 is mainly to protect the EL layer 46 from moisture, but it is also good if the second passivation film 49 is made to possess a heat radiation effect, similar to the first passivation film 41. The same materials as used for the first passivation film 41 can therefore be used as the formation material of the second passivation film 49. Note that when an organic material is used as the EL layer 46, it deteriorates due to bonding with oxygen, and therefore it is preferable to use an insulating film which does not easily emit oxygen.
  • Further, the EL layer is weak with respect to heat, as stated above, and therefore it is preferable to perform film deposition at a low temperature as possible (preferably in the range from room temperature to 120° C.). It can therefore be said that plasma CVD, sputtering, vacuum evaporation, ion plating, and solution application (spin coating) are desirable film deposition methods.
  • The EL display device of the present invention has a pixel portion containing a pixel with a structure as stated above, and TFTs having differing structures in response to their function are arranged in the pixel. A switching TFT having a sufficiently low off current value, and a current control TFT which is strong with respect to hot carrier injection can be formed within the same pixel, and an EL display device having high reliability and which is capable of good image display can thus be formed.
  • Note that the most important point in the pixel structure of FIG. 1 is that a multi-gate structure TFT is used as the switching TFT, and that it is not necessary to place limits on the structure of FIG. 1 with regard to such things as the placement of LDD regions.
  • A more detailed explanation of the present invention, having the above constitution, is now performed by the embodiments shown below.
  • Embodiment 1
  • The embodiments of the present invention are explained using FIGS. 3A to 5C. A method of manufacturing a pixel portion, and TFTs of a driver circuit portion formed in the periphery of the pixel portion, is explained here. Note that in order to simplify the explanation, a CMOS circuit is shown as a basic circuit for the driver circuits.
  • First, as shown in FIG. 3A, a base film 301 is formed with a 300 nm thickness on a glass substrate 300. Silicon oxynitride films are laminated as the base film 301 in embodiment 1. It is good to set the nitrogen concentration to between 10 and 25 wt % in the film contacting the glass substrate 300.
  • Further, it is effective to form a heat radiating layer, made from the same material as that of the first passivation film 41 shown in FIG. 1, as a portion of the base film 301. A large electric current flows in a current control TFT, heat is easily generated, and therefore it is effective to form the heat radiating layer as close as possible to the current control TFT.
  • Next, an amorphous silicon film (not shown in the figures) is formed with a thickness of 50 nm on the base film 301 by a known deposition method. Note that it is not necessary to limit this to the amorphous silicon film, and another film may be formed provided that it is a semiconductor film containing an amorphous structure (including a microcrystalline semiconductor film). In addition, a compound semiconductor film containing an amorphous structure, such as an amorphous silicon germanium film, may also be used. Further, the film thickness may be made from 20 to 100 nm.
  • The amorphous silicon film is then crystallized by a known method, forming a crystalline silicon film (also referred to as a polycrystalline silicon film or a polysilicon film) 302. Thermal crystallization using an electric furnace, laser annealing crystallization using a laser, and lamp annealing crystallization using an infrared lamp exist as known crystallization methods. Crystallization is performed in embodiment 1 using light from an excimer laser which uses XeCl gas.
  • Note that pulse emission type excimer laser light formed into a linear shape is used in embodiment 1, but a rectangular shape may also be used, and continuous emission argon laser light and continuous emission excimer laser light can also be used.
  • The crystalline silicon film is used as an active layer of the TFTs in embodiment 1, but it is also possible to use an amorphous silicon film as the active layer. However, it is necessary for a large current to flow through the current control TFT, and therefore it is more effective to use the crystalline silicon film, through which current easily flows.
  • Note that it is effective to form the active layer of the switching TFT, in which there is a necessity to reduce the off current, by the amorphous silicon film, and to form the active layer of the current control TFT by the crystalline silicon film. Electric current flows with difficulty in the amorphous silicon film because the carrier mobility is low, and the off current does not easily flow. In other words, the most can be made of the advantages of both the amorphous silicon film, through which current does not flow easily, and the crystalline silicon film, through which current easily flows.
  • Next, as shown in FIG. 3B, a protecting film 303 is formed on the crystalline silicon film 302 from a silicon oxide film having a thickness of 130 nm. This thickness may be chosen within the range of 100 to 200 nm (preferably between 130 and 170 nm). Furthermore, other films may also be used providing that they are insulating films containing silicon. The protecting film 303 is formed so that the crystalline silicon film is not directly exposed to plasma during addition of an impurity, and so that it is possible to have delicate concentration control of the impurity.
  • Resist masks 304 a and 304 b are then formed on the protecting film 303, and an impurity element which imparts n-type conductivity (hereafter referred to as an n-type impurity element) is added. Note that elements residing in periodic table group 15 are generally used as the n-type impurity element, and typically phosphorous or arsenic can be used. Note that a plasma doping method is used, in which phosphine (PH3) is plasma activated without separation of mass, and phosphorous is added at a concentration of 1×1018 atoms/cm3 in embodiment 1. An ion implantation method, in which separation of mass is performed, may also be used, of course.
  • The dose amount is regulated so that the n-type impurity element is contained in n-type impurity regions 305 and 306, thus formed by this process, at a concentration of 2×1016 to 5×1019 atoms/cm3 (typically between 5×1017 and 5×1018 atoms/cm3).
  • Next, as shown in FIG. 3C, the protecting film 303 is removed, and activation of the added periodic table group 15 element is performed. A known technique of activation may be used as the means of activation, and activation is done in embodiment 1 by irradiation of excimer laser light. Both of pulse emission type laser and a continuous emission type laser may be used, and it is not necessary to place any limits on the use of excimer laser light. The goal is the activation of the added impurity element, and it is preferable that irradiation is performed at an energy level at which the crystalline silicon film does not melt. Note that the laser irradiation may also be performed with the protecting film 303 in place.
  • Activation by heat treatment may also be performed along with activation of the impurity element by laser light. When activation is performed by heat treatment, considering the heat resistance of the substrate, it is good to perform heat treatment on the order of 450 to 550° C.
  • A boundary portion. (connecting portion) with regions along the edges of the n-type impurity regions 305 and 306, namely regions along the perimeter into which the n-type impurity element, which exists in the n-type impurity regions 305 and 306, is not added, is delineated by this process. This means that, at the point when the TFTs are later completed, extremely good connections can be formed between LDD regions and channel forming regions.
  • Unnecessary portions of the crystalline silicon film are removed next, as shown in FIG. 3D, and island shape semiconductor films (hereafter referred to as active layers) 307 to 310 are formed.
  • Then, as shown in FIG. 3E, a gate insulating film 311 is formed, covering the active layers 307 to 310. An insulating film containing silicon and with a thickness of 10 to 200 nm, preferably between 50 and 150 nm, may be used as the gate insulating film 311. A single layer structure or a lamination structure may be used. A 110 nm thick silicon oxynitride film is used in embodiment 1.
  • A conducting film with a thickness of 200 to 400 nm is formed next and patterned, forming gate electrodes 312 to 316. Note that in embodiment 1, the gate electrodes and lead wirings electrically connected to the gate electrodes (hereafter referred to as gate wirings) are formed from different materials. Specifically, a material having a lower resistance than that of the gate electrodes is used for the gate wirings. This is because a material which is capable of being micro-processed is used as the gate electrodes, and even if the gate wirings cannot be micro-processed, the material used for the wirings has low resistance. Of course, the gate electrodes and the gate wirings may also be formed from the same material.
  • Further, the gate wirings may be formed by a single layer conducting film, and when necessary, it is preferable to use a two layer or a three layer lamination film. All known conducting films can be used as the gate electrode material. However, as stated above, it is preferable to use a material which is capable of being micro-processed, specifically, a material which is capable of being patterned to a line width of 2 μm or less.
  • Typically, a film of a material chosen from among the group consisting of tantalum (Ta), titanium (Ti), molybdenum (Mo), tungsten (W), and chromium (Cr); or a nitrated compound of the above elements (typically a tantalum nitride film, a tungsten nitride film, or a titanium nitride film); or an alloy film of a combination of the above elements (typically a Mo—W alloy or a Mo—Ta alloy); or a silicide film of the above elements (typically a tungsten silicide film or a titanium silicide film); or a silicon film which has been made to possess conductivity can be used. A single layer film or a lamination may be used, of course.
  • A lamination film made from a 50 nm thick tantalum nitride (TaN) film and a 350 nm thick Ta film is used in embodiment 1. It is good to form this film by sputtering. Furthermore, if an inert gas such as Xe or Ne is added as a sputtering gas, then film peeling due to the stress can be prevented.
  • The gate electrodes 313 and 316 are formed at this time so as to overlap a portion of the n-type impurity regions 305 and 306, respectively, sandwiching the gate insulating film 311. This overlapping portion later becomes an LDD region overlapping the gate electrode.
  • Next, an n-type impurity element (phosphorous is used in embodiment 1) is added in a self-aligning manner with the gate electrodes 312 to 316 as masks, as shown in FIG. 4A. The addition is regulated so that phosphorous is added to impurity regions 317 to 323 thus formed at a concentration of {fraction (1/10)} to ½ that of the impurity regions 305 and 306 (typically between ¼and ⅓). Specifically, a concentration of 1×1016 to 5×1018 atoms/cm3 (typically 3×1017 to 3×1018 atoms/cm3) is preferable.
  • Resist masks 324 a to 324 d are formed next to cover the gate electrodes, as shown in FIG. 4B, and an n-type impurity element (phosphorous is used in embodiment 1) is added, forming impurity regions 325 to 331 containing a high concentration of phosphorous. Ion doping using phosphine (PH3) is also performed here, and is regulated so that the phosphorous concentration of these regions is from 1×1020 to 1×1021 atoms/cm3 (typically between 2×1020 and 5×1020 a toms/cm3).
  • A source region or a drain region of the n-channel TFT is formed by this process, and in the switching TFT, a portion of the n-type impurity regions 320 to 322 formed by the process of FIG. 4A remains. These remaining regions correspond to the LDD regions 15 a to 15 d of the switching TFT in FIG. 1.
  • Next, as shown in FIG. 4C, the resist masks 324 a to 324 d are removed, and a new resist mask 332 is formed. A p-type impurity element (boron is used in embodiment 1) is then added, forming impurity regions 333 and 334 containing a high concentration of boron. Boron is added here to a concentration of 3×1020 to 3×1021 atoms/cm3 (typically between 5×1020 and 1×1021 atoms/cm3) by ion doping using diborane (B2H6).
  • Note that phosphorous has already been added to the impurity regions 333 and 334 at a concentration of 1×1016 to 5×1018 atoms/cm3, but boron is added here at a concentration of at least 3 times that of the phosphorous. Therefore, the n-type impurity regions already formed completely invert to p-type, and function as p-type impurity regions.
  • Next, after removing the resist mask 332, the n-type and p-type impurity elements added at various concentrations are activated. Furnace annealing, laser annealing, or lamp annealing may be performed as a means of activation. Heat treatment is performed in embodiment 1 in a nitrogen atmosphere for 4 hours at 550° C. in an electric furnace.
  • It is important to remove as much of the oxygen in the atmosphere as possible at this time. This is because if any oxygen exists, then the exposed surface of the electrode oxidizes, inviting an increase in resistance, and at the same time it becomes more difficult to later make an ohmic contact. It is therefore preferable that the concentration of oxygen in the atmosphere in the above activation process be 1 ppm or less, desirably 0.1 ppm or less.
  • After the activation process is completed, a gate wiring 335 with a thickness of 300 nm is formed next. A metallic film having aluminum (Al) or copper (Cu) as its principal constituent (comprising 50 to 100% of the composition) may be used as the material of the gate wiring 335. As with the gate wiring 211 of FIG. 2, the gate wiring 335 is formed with a placement so that the gate electrodes 314 and 315 of the switching TFTs (corresponding to gate electrodes 19 a and 19 b of FIG. 2) are electrically connected. (See FIG. 4D.)
  • The wiring resistance of the gate wiring can be made extremely small by using this type of structure, and therefore a pixel display region (pixel portion) having a large surface area can be formed. Namely, the pixel structure of embodiment 1 is extremely effective because an EL display device having a screen size of a 10 inch diagonal or larger (in addition, a 30 inch or larger diagonal) is realized.
  • A first interlayer insulating film 336 is formed next, as shown in FIG. 5A. A single layer insulating film containing silicon is used as the first interlayer insulating film 336, but a lamination film may be combined in between Further, a film thickness of between 400 nm and 1.5 μm may be used. A lamination structure of an 800 nm thick silicon oxide film on a 200 nm thick silicon oxynitride film is used in embodiment 1.
  • In addition, heat treatment is performed for 1 to 12 hours at 300 to 450° C. in an atmosphere containing between 3 and 100% hydrogen, performing hydrogenation. This process is one of hydrogen termination of dangling bonds in the semiconductor film by hydrogen which is thermally activated. Plasma hydrogenation (using hydrogen activated by a plasma) may also be performed as another means of hydrogenation.
  • Note that the hydrogenation step may also be inserted during the formation of the first interlayer insulating film 336. Namely, hydrogen processing may be performed as above after forming the 200 nm thick silicon oxynitride film and then the remaining 800 nm thick silicon oxide film may be formed.
  • A contact hole is formed next in the first interlayer insulating film 336, and source wirings 337 to 340, and drain wirings 341 to 343 are formed. In embodiment 1, a lamination film with a three layer structure of a 100 nm titanium film, a 300 nm aluminum film containing titanium, and a 150 nm titanium film formed successively by sputtering, is used as these wirings. Other conducting films may also be used, of course, and an alloy film containing silver, palladium, and copper may also be used.
  • A first passivation film 344 is formed next with a thickness of 50 to 500 nm (typically between 200 and 300 nm). A 300 nm thick silicon oxynitride film is used as the first passivation film 344 in embodiment 1. This may also be substituted by a silicon nitride film. It is of course possible to use the same materials as those of the first passivation film 41 of FIG. 1.
  • Note that it is effective to perform plasma processing using a gas containing hydrogen such as H2 or NH3 before the formation of the silicon oxynitride film. Hydrogen activated by this preprocess is supplied to the first interlayer insulating film 336, and the film quality of the first passivation film 344 is improved by performing heat treatment. At the same time, the hydrogen added to the first interlayer insulating film 336 diffuses to the lower side, and the active layers can be hydrogenated effectively.
  • Next, as shown in FIG. 5B, a color filter 345 and a fluorescing body 346 are formed. Known materials may be used for these. Furthermore, they may be formed by being patterned separately, or they may be formed in succession and then patterned together. A method such as screen printing, ink jetting, or mask evaporation (a selective forming method using a mask material) may be used as the formation method.
  • The respective film thickness may be chosen in the range of 0.5 to 5 μm (typically between. 1 and 2 μm). In particular, the optimal film thickness of the fluorescing body 346 varies with the material used. In other words, if it is too thin, then the color transformation efficiency becomes poor, and if it is too thick, then the step becomes large and the amount of light transmitted drops. Optimal film thicknesses must therefore be set by taking a balance of both characteristics.
  • Note that, in embodiment 1, an example of a color changing method in which the light emitted from the EL layer is transformed in color, but if a method of manufacturing individual EL layers which correspond to R, G, and B, is employed, then the color filter and the fluorescing body can be omitted.
  • A second interlayer insulating film 347 is formed next from an organic resin. Materials such as polyimide, polyamide, acrylic, and BCB (benzocyclobutene) can be used as the organic resin. In particular, the purpose of being a leveling film is strong in the second interlayer insulating film 347, and therefore acrylic, having superior leveling characteristics, is preferable. An acrylic film is formed in embodiment 1 with a film thickness which can sufficiently level the step between the color filter 345 and the fluorescing body 346. This thickness is preferably from 1 to 5 μm (more preferably between 2 and 4 μm).
  • A contact hole for reaching the drain wiring 343 is formed next in the second interlayer insulating film 347 and in the first passivation film 344, and a pixel electrode 348 is formed. A compound of indium oxide and tin oxide is formed into 110 nm thick in embodiment 1, and patterning is performed, making the pixel electrode. The pixel electrode 348 becomes an anode of the EL element. Note that it is also possible to use other materials: a compound film of indium oxide and zinc oxide, or a zinc oxide film containing gallium oxide.
  • Note that embodiment 1 becomes a structure in which the pixel electrode 348 is electrically connected to the drain region 331 of the current control TFT, through the drain wiring 343. This structure has the following advantages.
  • The pixel electrode 348 becomes directly connected to an organic material such as the EL layer (emitting layer) or a charge transporting layer, and therefore it is possible for the mobile ions contained in the EL layer to diffuse throughout the pixel electrode. In other words, without connecting the pixel electrode 348 directly to the drain region 331, a portion of the active layer, the introduction of mobile ions into the active layer due to the drain wiring 343 being interrupted can